The present invention relates to cryogenic gas plants and particularly to gas plants which are designed to separate natural gas liquids (NGL), which contain ethane and higher boiling hydrocarbons from a natural gas stream. The evolution of cryogenic gas plants is reviewed in a paper presented at the 1977 Gas Processing Association Convention entitled "Evolution in Design" by R. L. McKee. This paper describes the use of turbo-expanders to increase the thermodynamic efficiency of the gas plant and thus improve its overall economics. A similar type of cryogenic system for recovering ethane and higher boiling hydrocarbons from a natural gas stream is described in U.S. Pat. No. 3,292,380. A more recent description of cryogenic gas processing plants appears in the July 14, 1980 edition of the "Oil and Gas Journal" at page 76 et. seq. All of these references describe the use of a turbo-expander for supplying the feedstock to a demethanizing column where the methane is separated from the ethane and the higher boiling hydrocarbons.
While these systems have been satisfactory, they do not recover all the ethane and higher boiling point liquids. The ethane and higher boiling point liquids are used as chemical feedstocks which have more value in today's markets as chemical feedstocks than as natural gas. The lack of complete recovery of the ethane partially is due to the relative volatility of methane to ethane and to the fact that a considerable amount of the ethane is contained as a vapor in the methane when it enters the demethanizing column. The portion of the ethane contained as vapor remains in the gas phase and is discharged from the top of the column as pipeline gas.
A further problem arises when attempts are made to operate plants near their capacity or beyond. When operating plants at or above their capacity, flooding of the demethanizing column with liquid occurs. When this occurs, additional ethane and higher boiling point liquids are lost to the natural gas stream instead of being condensed and removed as liquid from the bottom of the column.